Closes #1443. setnames accepts -ve indices in 'old' arg.

R/data.table.R

@@ -2196,11 +2196,12 @@ setnames <- function(x,old,new) {
     } else {
         if (missing(old)) stop("When 'new' is provided, 'old' must be provided too")
         if (!is.character(new)) stop("'new' is not a character vector")
-        if (length(new)!=length(old)) stop("'old' is length ",length(old)," but 'new' is length ",length(new))
         if (is.numeric(old)) {
-            tt = old<1L | old>length(x) | is.na(old)
+	    if (length(sgn <- unique(sign(old))) != 1L)
+		stop("Items of 'old' is numeric but has both +ve and -ve indices.")
+	    tt = abs(old)<1L | abs(old)>length(x) | is.na(old)
             if (any(tt)) stop("Items of 'old' either NA or outside range [1,",length(x),"]: ",paste(old[tt],collapse=","))
-            i = as.integer(old)
+	    i = if (sgn == 1L) as.integer(old) else seq_along(x)[as.integer(old)]
             if (any(duplicated(i))) stop("Some duplicates exist in 'old': ",paste(i[duplicated(i)],collapse=","))
         } else {
             if (!is.character(old)) stop("'old' is type ",typeof(old)," but should be integer, double or character")
@@ -2209,6 +2210,7 @@ setnames <- function(x,old,new) {
             if (any(is.na(i))) stop("Items of 'old' not found in column names: ",paste(old[is.na(i)],collapse=","))
             if (any(tt<-!is.na(chmatch(old,names(x)[-i])))) stop("Some items of 'old' are duplicated (ambiguous) in column names: ",paste(old[tt],collapse=","))
         }
+	if (length(new)!=length(i)) stop("'old' is length ",length(old)," but 'new' is length ",length(new))
     }
     # update the key if the column name being change is in the key
     m = chmatch(names(x)[i], key(x))

README.md

@@ -45,6 +45,8 @@
 
   15. `fread` gains `key` argument, [#590](https://github.com/Rdatatable/data.table/issues/590).
 
+  16. `setnames` accepts negative indices in 'old' argument, [#1443](https://github.com/Rdatatable/data.table/issues/1443). Thanks @richierocks.
+
 #### BUG FIXES
 
   1. Now compiles and runs on IBM AIX gcc. Thanks to Vinh Nguyen for investigation and testing, [#1351](https://github.com/Rdatatable/data.table/issues/1351).

inst/tests/tests.Rraw

@@ -7336,6 +7336,11 @@ ans = data.table(id=rep(c("ID_1","ID_2","ID_3"), c(2,1,4)),
                dens=c(2331,1644,1890,8000,1452,1596,7521))
 test(1591, rbindlist(mylist, idcol="id"), ans)
 
+# FR #1443
+DT <- data.table(x = 1:3, y = 4:6, z = 7:9)
+test(1592.1, setnames(DT, -5, "bla"), error="Items of 'old'")
+test(1592.2, names(setnames(DT, -1, c("m", "n"))), c("x", "m", "n"))
+
 ##########################
 
 # TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.

man/setattr.Rd

@@ -14,7 +14,7 @@ setnames(x,old,new)
   \item{name}{ The character attribute name. }
   \item{value}{ The value to assign to the attribute or \code{NULL} removes the attribute, if present. }
   \item{old}{ When \code{new} is provided, character names or numeric positions of column names to change. When \code{new} is not provided, the new column names, which must be the same length as the number of columns. See examples. }
-  \item{new}{ Optional. New column names, the same length as \code{old}. } 
+  \item{new}{ Optional. New column names, must be the same length as columns provided to \code{old} argument. } 
 }
 \details{
 
@@ -49,6 +49,9 @@ setnames(DT,2:3,c("D","E"))        # multiple
 setnames(DT,c("a","E"),c("A","F")) # multiple by name (warning if either "a" or "E" is missing)
 setnames(DT,c("X","Y","Z"))        # replace all (length of names must be == ncol(DT))
 
+DT <- data.table(x = 1:3, y = 4:6, z = 7:9)
+setnames(DT, -2, c("a", "b"))      # NEW FR #1443, allows -ve indices in 'old' argument
+
 DT = data.table(a=1:3, b=4:6)
 f = function(...) {
     # ...